A H M 531 The Civil Engineering Center

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Everett Floyd
6 years ago
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1 Objective: To calculated the unconfined compressive strength for a cohesion sample. General discussion: Unconfined compression test is a special case of the tri-axial compression test, where the lateral stress will equal to zero as no confinement exists in that direction, so the main stresses are applied mainly on the vertical direction of the sample. The results obtained from the unconfined test are only approximation of the actual soil shear strength for the following reasons; 1. Sampling disturbance during recovery. 2. Soil in field is affected by lateral restraints provided by the surrounding soil mass. But it is widely used due to its simplicity and low cost. The unconfined compression test is applied only to cohesive soil as it sticks together even without lateral confinement. Cohesion less samples would simply fall apart. The used sample must meet several conditions to be appropriate for the test: 1. The length to diameter ratio must be between 2 and 2.5 as specified by ASTM. 2. The surface of the specimen must be uniformly shaped. 3. The soil shall be homogenous and contains no obstacle such as gravel particles. 4. It shall contain no desiccation cracks. From this test we aim to graph the stress-strain diagram of the tested soil, and use that to obtain the average unconfined peak strength q u rather than using the largest value of stress from the computation sheet. 1Visit for more lab reports and lecture notes!

2 The design of structures uses the average peak strength of the soil unless the corresponding strain for that peak is more than 15%, where ASTM specifies this value of strain as a maximum, and in this case the design stress is taken to be the stress corresponding to the 15% strain on the diagram. The results necessary to obtain the stress-strain diagram are obtained by either a strain controlled test, where the values of strain are known and the corresponding stresses are measured. Or by stress controlled test, where the applied stress is known and the corresponding strain is measured. The strain controlled test is more common and widely used whereas the stress controlled test is seldom used as it may result erratic strain response and the ultimate strength falling between two points of loading. There are two devices that are used to measure the unconfined compressive strength; Portable device test this provides fast and approximate results, where the specimen is placed between two jaws; the upper is fixed and the lower is moving and connected A to H a spring M 531 by which the load is applied by a rate The enough Civil to break the Engineering specimen during two Center minutes. Suitable stiffness of the spring shall be used in accordance to the soil strength. By a pencil attached to the device, the stress-deformation diagram is plotted on a paper stuck to the back of the device, the diagram will appear upside down. From the peak point, the maximum deformation ΔL is used to find the strain; The unconfined shear strength is measured by using the formula; Compression device test 2Visit for more lab reports and lecture notes!

3 the device that was available in the laboratory was strain controlled device, so all the necessary information was the stresses corresponding to each strain. The stresses are measured by means of using a grooving ring containing a dial clock, when the load is applied to the ring,which rests on the specimen by a cylindrical metal to distribute the load uniformly to the specimen, splaying occurs and that increases the dial reading. Every grooving ring has a calibration chart that is used to convert the dial reading into loads, it is preferred to graph the relation of the dial reading to load for each ring before using it, and then find the slope of the chart for which the slope is called Load Ring Constant L.R.C. and this value can be used to find any load that corresponds to any dial reading the ring by the formula; It is important to know that every grooving ring has a maximum load that it handles, don t exceed that! A suitable ring that will A give H M an obvious 531reading for the sample, depending The on Civil the strength Engineering of the soil, must be Center chosen. After calculating the load corresponding to each strain value, the stresses are calculated by dividing the load over the area. The area used in the formula shall be the corrected area which is obtained by the formula; This area correction is to make some allowance for the way the soil is actually being loaded in the field. Applying a correction to the 3Visit for more lab reports and lecture notes!

4 original sample area is also somewhat conservative since the ultimate strength computed this way is less than that computed from the original area. The test continues till either elastic or non-elastic failure occurs, the non-elastic failure of the sample is known visually, whereas the elastic failure is known when the dial reading remains constant after three records. Sample identification: Clayey cohesive sample of 76mm length and 38mm diameter. Equipments: 1. Unconfined compressive testing machine. 2. Grooving ring with dial clock. 3. Water content cans. 4. Weighing machine. 5. Portable compression device. 6. Timer. Procedures: 4Visit for more lab reports and lecture notes!

5 Portable device test: 1. The sample was placed between the two jaws, and a graphing paper was fixed on the back of the device. And the pencil was placed on the zero point. 2. The load was applied within a rate so that the sample broke within two minutes. 3. Δ was obtained by using the mask and the obtained graph. Compression testing machine: 1. After measuring the samples length and diameter, the sample was placed into the machine. 2. The stain rate was entered to the machine to be 1mm / min. 3. The load was applied and the dial reading was recorded every 15seconds. 4. Recording the dial reading was continued till elastic failure was noticed. 5. A portion of the wet sample was weighed and place in the oven for 24 hours, and the dry weight was recorded. Calculations: 5Visit for more lab reports and lecture notes!

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7 Discussion and results: The obtained unconfined compressive strength 71kPa corresponded to a strain of which is less than 15% of the total strain. By comparing the stress obtained by the portable device and the compression device, we can see that the portable device result is near to the compression device one and can be used as an approximation. In our results some errors might have occurred from the unstable rate of applying the load in the portable device. 7Visit for more lab reports and lecture notes!

Unconfined compression strength Instrument Set Up

Unconfined compression strength Instrument Set Up INTRODUCTION The main object of the test is to determine the unconfined compressive strength (qu) of a cohesive soil. The test is suitable for saturated